A HIRF can Hurt Your Aircraft Gear – Testing High Intensity Radiated Field Immunity

Air travel is the safest form of travel, by far. Data from the International Air Transport Association (IATA) showed that the risk is so low that that on average, a person would need to fly every day for 461 years before experiencing an accident with at least one fatality.

The aviation industry has held its impressive record through careful attention to detail. That attention is focused on the aircraft itself, of course, but also on understanding the aircraft’s environment. Besides the obvious atmospheric concerns like wind, rain, and lightning, the presence of radiofrequency (RF) fields can disrupt the aircraft’s electronics.

RF fields are everywhere, and most are at low enough levels that they pose little threat to safe operation. But high intensity radiated fields (HIRFs) can overwhelm guidance devices. Airports are rich with HIRFs from radar, guidance, and communications systems that rely on high-powered transmitters. Elite Electronic Engineering’s Pat Hall and Tom Klouda have been performing HIRF tests on aircraft components for decades and explain how the test is done.

Standards and Test Planning

Aviation HIRF testing is specified in the Radio Technical Commission for Aeronautics (RTCA) standard DO-160, Section 20. The standard identifies susceptibility categories set at different RF levels. The table below shows the categories in the columns and the frequency ranges in the rows. The cells of the table give the test levels in Volts/meter.

Testing is normally done in one of Elite’s mode-stirred chambers, which are shielded rooms equipped with rotating stirrers. Different forms of the RF field are applied, such as pulsed or continuous wave, depending on the application.

As an example, Elite’s lab often tests aircraft display hardware, which is composed of the display panel itself and the electronics that drive it. In those cases, Category G is the level most often called for when testing those devices. The specified field levels are highlighted below, taken from Table 20-3 of the DO-160 standard. The field at those levels is applied to the equipment under test (EUT) while its operation is monitored for responses.

radiated susceptibility test levels table

Those fields are generated most often in a mode-stirred chamber, shown in the illustration below. An RF amplifier feeds an antenna inside a shielded enclosure to create the field, and the modes of the field are stirred by a rotating metal tuner/stirrer. The effect is to provide a consistent average field level to the the EUT from the combination of reflections from the metal surfaces and the paddle’s rotation. The wide variety of angles and levels seen by the EUT during the test assures that specified overall level over time will be applied.

Stirred-mode test chamber setup

Stirred-mode test chamber setup, showing the EUT within the calibrated test volume and the tuner/stirrer that provides an overall average field level

The test chamber calibration establishes the power levels needed to generate the field across the frequency range. The dashed-line box in the illustration below shows the chamber’s test volume. An isotropic RF probe measures the field level at each frequency in the specified range. The probe is set up at nine points in the test volume, one at each corner and one in the center. The power-level numbers collected in calibration are programmed into the amplifier controller, which can then provide consistent field levels during the test.

test volume diagram

Stirred-mode test chamber setup, showing the EUT within the calibrated test volume and the tuner/stirrer that provides an overall average field level

The EUT is set up according to its test plan and monitored for any response as the radiated field is applied across the frequency range. The positioning of enclosures, cables, connectors, and other components of the EUT are specified in the test plan so that its actual environment is simulated. The EUT’s function and form and its proximity to other equipment and the aircraft’s body are fundamental to determining how to position it during the test. The EUT’s pass/fail criteria also need to be understood so that meaningful evaluations can be made if responses are seen during the test.

Elite has two mode-stirred chambers with different test volumes. The larger of the two can test from 100 MHz – 18 GHz up to 2kV/m, and the smaller can test from 400 MHz – 18 GHz up to 5kV/m. The photo below shows Elite HIRF expert Tom Klouda setting up a test in the larger chamber, with the tuner/stirrer visible in the background.

Elite's team reviews chamber setup.

Elite’s Tom Klouda (center) reviews chamber setup with Mark Rugg (left) and Fred Rub. The mode-stirring paddle is at the upper rear of the chamber.

Preparation takes up the bulk of time for a test. EUTs can be any size, with a wide variety of ancillary equipment and cables that collectively make up the overall EUT. The test plan will specify how the EUT is to be configured, how the cables are to be exposed, and what modes of operation the EUT needs to run. With those factors in place and the EUT in position, the actual test is run across the specified range while the EUT is monitored.

Contact the experts at Elite with any questions on HIRF testing, the applicable standard, and the steps required to prepare. Trust Elite put its decades of experience to work for you.